I'm not even sure this is a bug or intentional behavior, but I haven't found any documentation on it, so I think it's probably a good idea to document this somewhere, and the official forum doesn't seem like a bad place.
I also haven't filed a bug on it yet, since I'm not even sure it is a bug so far.
I'm using Windows 7/64 and current trunk builds (at least up to 20100205) are unable to do hardware accelerated WebGL by just setting webgl.enabled_for_all_sites. Instead, you have to provide the path to the system OpenGL library via webgl.osmesalib. This could be because the opengl32.dll doesn't reside in the system32 directory on 64bit editions of Windows ... or something else, I don't know. Setting it to the SysWOW64 folder instead fixes the issue for me (i.e. C:\Windows\SysWOW64\opengl32.dll)
Can anybody shed some light on this?